Compositions for enhanced fracture cleanup using redox treatment

ABSTRACT

A cleanup fluid for reducing a viscosity of a residual viscous material in fractures of a hydrocarbon-bearing formation is disclosed. The cleanup fluid includes an acid precursor, the acid precursor operable to trigger an exothermic reaction component and the exothermic reaction component operable to generate heat, where the heat is operable to reduce a viscosity of the residual viscous material to create a reduced viscosity material, the reduced viscosity material operable to flow from the fractures.

PRIORITY

This application is a continuation-in-part application of and claims priority to and the benefit of the application published as U.S. Pub. No. 2015/0300142 A1, filed Apr. 17, 2015, which claims priority to and the benefit of U.S. Provisional Patent Application No. 61/980,664, filed Apr. 17, 2014, the entire disclosures of which is hereby expressly incorporated here by reference.

FIELD

This disclosure relates to a composition and method to improve the recovery of hydrocarbons from a fractured formation. More specifically, this disclosure relates to a composition and method to reduce the viscosity of a fracturing fluid or other undesired viscous fluid present in a wellbore or hydrocarbon-bearing reservoir.

BACKGROUND

Hydraulic fracturing fluids containing proppants are used extensively to enhance productivity from hydrocarbon-bearing reservoir formations, including carbonate and sandstone formations. During hydraulic fracturing operations, a fracturing treatment fluid is pumped under a pressure and rate sufficient for cracking the formation of the reservoir and creating a fracture. Fracturing operations usually consist of three main stages including a pad fluid stage, a proppant fluid stage, and an overflush fluid stage. The pad fluid stage typically consists of pumping a pad fluid into the formation. The pad fluid is a viscous gelled fluid which initiates and propagates the fractures. Auxiliary fractures can propagate from the fractures to create fracture networks. A fracture network can comprise fractures and auxiliary fractures. Auxiliary fractures can connect the fractures.

The proppant fluid stage involves pumping a proppant fluid into the fractures of the formation. The proppant fluid contains proppants mixed with a viscous gelled fluid or a visco-elastic surfactant fluid. The proppants in the proppant fluid are lodged in the fractures and create conductive fractures through which hydrocarbons flow. The final stage, the overflush stage, includes pumping a viscous, gelled fluid into the fractures to ensure the proppant fluid is pushed inside the fractures. While the three stages have different aims, all three make use of highly viscous fluids, in addition to or alternative to gelled fluids, to achieve those aims.

A downside of the traditional method is that a high volume of gelled or polymeric materials can be left behind in the fractures. The gelled materials can be concentrated around the proppant in the fractures or can be freely located in the fractures. The gelled material acts to block the fractures reducing the fracture conductivity. The hydrocarbons which flow from the reservoir formation are unable to move the gelled materials. Traditional methods for cleaning the fractures involve viscosity breakers or other elements to breakdown the viscous fracturing fluids. These traditional methods suffer from an inability to completely cleanup the fractures, leaving residual viscous material and reduced conductivity.

SUMMARY

This disclosure relates to a composition and method to improve the recovery of hydrocarbons from a fractured formation. More specifically, this disclosure relates to a composition and method to reduce the viscosity of a fracturing fluid or a blockage material, such as, for example, a gelled or viscous fracturing fluid, or asphaltenes, or a similar oily sludge.

In one aspect, a method for improved hydrocarbon recovery from a formation due to cleanup of a residual viscous material is provided. The method includes the step of fracturing the formation with a fracturing fluid to generate fractures. The fracturing fluid includes a viscous fluid component, the viscous fluid component operable to fracture the formation to create fractures leaving behind the residual viscous material in the fractures, the viscous fluid component having a viscosity, a proppant component, the proppant component includes a proppant, the proppant operable to hold open the fractures, where the proppant component is carried to the fractures by the viscous fluid component, and a cleanup fluid.

The cleanup fluid includes an acid precursor, the acid precursor operable to trigger an exothermic reaction component, and the exothermic reaction component operable to generate heat, where the heat is operable to reduce a viscosity of the residual viscous material to create a reduced viscosity material, the reduced viscosity material operable to flow from the formation. Fractures can include auxiliary fractures, which propagate from the fractures.

In certain aspects, the exothermic reaction component includes an ammonium containing compound and a nitrite containing compound. In certain aspects of the present disclosure, the ammonium containing compound is NH₄Cl and the nitrite containing compound is NaNO₂. In certain aspects of the disclosure, the acid precursor is triacetin.

In a second aspect of the present disclosure, a cleanup fluid for reducing a viscosity of a residual viscous material in fractures is provided. The cleanup fluid includes an acid precursor, the acid precursor operable to trigger an exothermic reaction component, and the exothermic reaction component operable to generate heat, where the heat is operable to reduce a viscosity of the residual viscous material to create a reduced viscosity material, the reduced viscosity material operable to flow from the fractures.

In certain aspects, the exothermic reaction component includes an ammonium containing compound and a nitrite containing compound. In certain aspects of the present disclosure, the ammonium containing compound is NH₄Cl and the nitrite containing compound is NaNO₂. In certain aspects of the present disclosure, the acid precursor is triacetin.

In a third aspect, a method to cleanup fractures post hydraulic fracturing is provided. The method includes the steps of fracturing a formation in a hydraulic fracturing operation to produce fractures, and injecting a cleanup fluid into the fractures to reduce a viscosity of a residual viscous material.

In certain aspects of the present disclosure, the step of fracturing the formation includes the step of fracturing the formation with a fracturing fluid to generate fractures. The fracturing fluid includes a viscous fluid component, the viscous fluid component operable to fracture the formation to create fractures leaving behind the residual viscous material in the fractures, the viscous fluid component having a viscosity, and a proppant component, the proppant component comprising a proppant, the proppant operable to hold open the fractures, where the proppant component is carried to the fractures by the viscous fluid component. In certain aspects of the present disclosure, the cleanup fluid includes an acid precursor, the acid precursor operable to trigger an exothermic reaction component, and the exothermic reaction component operable to generate heat, where the heat is operable to reduce a viscosity of the residual viscous material to create a reduced viscosity material, the reduced viscosity material operable to flow from the fractures. In certain aspects of the present disclosure, the exothermic reaction component includes an ammonium containing compound and a nitrite containing compound. In certain aspects, the ammonium containing compound is NH₄Cl and the nitrite containing compound is NaNO₂. In certain aspects, the acid precursor is triacetin.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following descriptions, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the disclosure and are therefore not to be considered limiting of the disclosure's scope as it can admit to other equally effective embodiments.

FIG. 1 is a graphic representation of the effect of the cleanup fluid on the viscosity of a residual viscous material.

FIG. 2 is a graphic representation of the heat and pressure generated by an exothermic reaction component.

FIG. 3a is a pictorial representation of the residual viscous material before the reaction of an exothermic reaction component of the cleanup fluid.

FIG. 3b is a pictorial representation of the residual viscous material after the reaction of an exothermic reaction component of the cleanup fluid.

FIG. 4 is a graphic representation of the effect of the reaction of the exothermic reaction component on the viscosity of a fracturing fluid.

FIG. 5a is a pictorial representation of blockage materials collected from an injection well.

FIG. 5b is a pictorial representation of blockage materials collected from an injection well.

FIG. 6 is a graph showing the effect of an exothermic reaction component on the temperature of blockage materials from the injection well in Example 4.

FIG. 7 is a graph showing the effect of an exothermic reaction component on the viscosity of blockage materials from the injection well in Example 4.

FIG. 8 is a graph showing the effect of an exothermic reaction treatment used to treat the blocked wellbore of Example 4.

FIG. 9 is a graph showing the effect of an exothermic reaction treatment used to treat the rock matrix surrounding the wellbore of Example 4.

FIG. 10 is a graph showing the pre-exothermic reaction component injectivity and post-exothermic reaction component injectivity of the injection well of Example 4.

DETAILED DESCRIPTION

While the disclosure will be described with several embodiments, it is understood that one of ordinary skill in the relevant art will appreciate that many examples, variations and alterations to the apparatus and methods described here are within the scope and spirit of the disclosure. Accordingly, the embodiments described here are set forth without any loss of generality, and without imposing limitations, on the claims.

In one aspect, a method for improved hydrocarbon recovery from a formation due to cleanup of a residual viscous material is provided. The hydraulic fracturing operation fractures the formation using fracturing fluid to create fractures. Formations include sandstone and carbonate, for example.

The fracturing fluid includes a viscous fluid component and a proppant component. The viscous fluid component has a viscosity. The viscous fluid component is operable to increase the viscosity of the fracturing fluid. Viscous fluid components include viscosified water-based fluids, non-viscosified water-based fluids, gel-based fluids, gel oil-based fluids, acid-based fluids, and foam fluids. Gel-based fluids include cellulose derivatives and guar-based fluids. Cellulose derivatives include carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxypropyl cellulose, and methyl hydroxyl ethyl cellulose.

Guar-based fluids include hydroxypropyl guar, carboxymethyl guar, guar cross-linked boron ions from an aqueous borax/boric acid solution, and guar cross-linked with organometallic compounds. Organometallic compounds include zirconium, chromium, antimony, and titanium salts. Gel oil-based fluids include aluminum phosphate-ester oil gels. In at least one embodiment of the present disclosure, the viscous fluid component is an aqueous guar solution, having a concentration of guar gum between about 0.1% and about 15%, between about 0.1% and about 10%, between about 1% and about 10%, between about 2% and about 8%, and between about 4% and about 6%.

The proppant component includes a proppant. The proppant is operable to hold open fractures created by the viscous fluid component. Any proppants capable of holding open fractures to create a conductive fractures are suitable for use in the present disclosure. In some embodiments, the proppant component includes a viscous carrier fluid having a viscosity. Viscous carrier fluids include viscosified water-based fluids, non-viscosified water-based fluids, gel-based fluids, gel oil-based fluids, acid-based fluids, and foam fluids. Gel-based fluids include cellulose derivatives and guar-based fluids. Cellulose derivatives include carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxypropyl cellulose, and methyl hydroxyl ethyl cellulose.

Guar-based fluids include hydroxypropyl guar, carboxymethyl guar, guar cross-linked boron ions from an aqueous borax/boric acid solution, and guar cross-linked with organometallic compounds. Organometallic compounds include zirconium, chromium, antimony, and titanium salts. Gel oil-based fluids include aluminum phosphate-ester oil gels. In some embodiments, the hydraulic fracturing operation uses a one stage fracturing fluid, in which the fracturing fluid includes both the viscous fluid component and the proppant component, in which the viscous fluid component carries the proppant component to the fractures.

In at least one embodiment, the hydraulic fracturing operation uses a multi-stage fracturing fluid in which the viscous fluid component is injected into the formation, followed by the proppant component in the viscous carrier fluid. In some embodiments, the injection of the proppant component is followed by injection of additional viscous fluids to ensure the proppants are placed in the fractures. The additional viscous fluids have a viscosity.

In some embodiments, the viscosity of the viscous fluid component, the viscous carrier fluid, and additional viscous fluids are the same. In some embodiments, the viscosity of the viscous fluid component, the viscous carrier fluid, and additional viscous fluids are different. The injection of the fracturing fluid ceases after the proppants are placed in the fractures and the fracturing fluid is allowed to seep from the fractures. In some embodiments, the injection of the hydraulic fracturing fluid including the viscous fluid component in addition to or alternative to the proppant component in addition to or alternative to the overflush component in addition to or alternative to the exothermic reaction component does not generate foam or introduce foam into the hydraulic formation including the hydraulic fractures.

The hydraulic fracturing operation can leave residual viscous material in the fractures of a hydraulic formation. Residual viscous materials can include carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxypropyl cellulose, methyl hydroxyl ethyl cellulose, guar gum, hydroxypropyl guar, carboxymethyl guar, guar cross-linked with boron, aluminum phosphate-ester oil gel, and guar cross-linked with organometallic compounds. Organometallic compounds include zirconium, chromium, antimony, and titanium salts. In some embodiments of the present disclosure, the residual viscous material is a gelled material. In some embodiments of the present disclosure, the residual viscous material is a polymeric material. In at least one embodiment of the present disclosure, the residual viscous material is guar gum. The residual viscous material has a viscosity greater than the fracturing fluid. In at least one embodiment of the present disclosure, the residual viscous material is surrounding or adjacent to the proppants placed in the fractures.

The cleanup fluid acts, after the proppants have been placed in the fractures, to remove the residual viscous material. In one embodiment, the cleanup fluid is mixed with the fracturing fluid. In at least one embodiment of the present disclosure, where a multi-stage fracturing fluid is used, the cleanup fluid is a component of the fluids used at each stage of the hydraulic fracturing operation. In an alternate embodiment, the cleanup fluid is added only to the fluid of the final stage of the hydraulic fracturing operation, such as, for example, the overflush stage. In some embodiments, the cleanup fluid is pumped to the fractured formation as a separate step following the hydraulic fracturing operation.

In some embodiments, the cleanup fluid includes an acid precursor and an exothermic reaction component. The reaction of the exothermic reaction component results in a release of kinetic energy and thermal energy. The reaction of the exothermic reaction component generates heat and increases the pressure. The generated heat increases the temperature of the surrounding fluids, including fracturing fluid remaining in the fractures and residual viscous material. The increase in temperature reduces the viscosity of the fracturing fluid. The increase in temperature reduces the viscosity of the residual viscous material left in the fractures to create a reduced viscosity material. The reduced viscosity material flows from the fractures of the formation to the wellbore. The increase in pressure provides lift energy to push the reduced viscosity materials through the wellbore toward the surface. The removal of the residual viscous material increases the conductivity of the fractures. Increased conductivity of the fractures increases seepage of the fracturing fluid, improves fracturing efficiency, minimizes need for additional fracturing jobs, minimizes time between fracturing and well production, and increases hydrocarbon flow, which translates to increased hydrocarbon recovery.

The acid precursor is any acid that releases hydrogen ions to trigger the reaction of the exothermic reaction component. Acid precursors include triacetin (1,2,3-triacetoxypropane), methyl acetate, HCl, and acetic acid. In at least one embodiment, the acid precursor is triacetin. In at least one embodiment, the acid precursor is acetic acid.

The exothermic reaction component includes one or more redox reactants that exothermically react to produce heat and increase pressure. Exothermic reaction components include urea, sodium hypochlorite, ammonium containing compounds, and nitrite containing compounds. In at least one embodiment of the present disclosure, the exothermic reaction component includes ammonium containing compounds. Ammonium containing compounds include ammonium chloride, ammonium bromide, ammonium nitrate, ammonium sulfate, ammonium carbonate, and ammonium hydroxide.

In at least one embodiment, the exothermic reaction component includes nitrite containing compounds. Nitrite containing compounds include sodium nitrite and potassium nitrite. In at least one embodiment, the exothermic reaction component includes both ammonium containing compounds and nitrite containing compounds. In at least one embodiment, the ammonium containing compound is ammonium chloride, NH₄Cl. In at least one embodiment, the nitrite containing compound is sodium nitrite, NaNO₂.

In at least one embodiment of the present disclosure, the exothermic reaction component includes two redox reactants: NH₄Cl and NaNO₂, which react according to the following:

In a reaction of the exothermic reaction components according to the previous equation, generated gas and heat contribute to the reduction of the viscosity of the residual viscous material.

The exothermic reaction component is triggered to react. In at least one embodiment of the present disclosure, the exothermic reaction component is triggered within the fractures. In at least one embodiment of the present disclosure, the acid precursor triggers the exothermic reaction component to react by releasing hydrogen ions.

In at least one embodiment, the exothermic reaction component is triggered by heat. The wellbore temperature is reduced during a pre-pad injection or a pre-flush with brine and reaches a temperature less than 120° F. (48.9° C.). The fracturing fluid of the present disclosure is then injected into the well and the wellbore temperature increases. When the wellbore temperatures reaches a temperature greater than or equal to 120° F., the reaction of the redox reactants is triggered. In at least one embodiment, the reaction of the redox reactants is triggered by temperature in the absence of the acid precursor. In at least one embodiment, the exothermic reaction component is triggered by heat when the exothermic reaction component is within the fractures.

In at least one embodiment, the exothermic reaction component is triggered by pH. A base is added to the fracturing fluid of the present disclosure to adjust the pH to between 9 and 12. In at least one embodiment, the base is potassium hydroxide. The fracturing fluid with the base is injected into the formation. Following the injection of the fracturing fluid, an acid is injected to adjust the pH to less than 6. When the pH is less than 6, the reaction of the redox reactants is triggered. In at least one embodiment, the exothermic reaction component is triggered by pH when the exothermic reaction component is within the fractures.

In at least one embodiment of the present disclosure, the cleanup fluid is introduced to the fractures following the hydraulic fracturing operation. Dual-string coiled tubing is used to introduce the exothermic reaction component and the acid precursor to the wellbore. In at least one embodiment, the exothermic reaction component includes NH₄Cl and NaNO₂. The acid precursor is acetic acid. The acetic acid is mixed with NH₄Cl and injected in parallel with the NaNO₂, using different sides of the dual-string coiled tubing. The exothermic reaction component and the acid precursor mix within the fractures.

EXAMPLES Example 1

An exothermic reaction component of a cleanup fluid consisting of 3M NH₄Cl and 3M NaNO₂ was added to a solution of 1% by volume guar at room temperature, see FIG. 3. The exothermic reaction component was triggered by heat. The viscosity of the solution was measured before, during, and after the reaction using a Chandler viscometer. Prior to reaction of the exothermic reaction component, the viscosity of the residual viscous material was 85 centipoise (cP). FIG. 1 is a graph of the viscosity following the reaction of the exothermic reaction component. The graph shows that the viscosity of the residual viscous material was reduced to less than 8.5 cP. FIG. 3b shows the solution, including the residual viscous material after the reaction of the exothermic reaction component.

Example 2

A solution of an exothermic reaction component was prepared from 3M NH₄Cl and 3M NaNO₂. The solution was placed in an autoclave reactor at room temperature and an initial pressure of 1,000 psi. The reactor temperature was increased. The reaction was triggered at about 120° F., see FIG. 2. Due to the reaction, the temperature in the reactor reached a temperature of 545° F. and a pressure of 3,378 psi, see FIG. 2.

Example 3

The exothermic reaction component showed compatibility with the viscous fluid component (here a cross-linked gel). The fracturing fluid with the viscous fluid component, the exothermic reaction component, and the proppant component was also prepared and showed compatibility. The fracturing fluid, without the proppant component, was activated in the autoclave reactor by heating to the wellbore temperature to trigger the reaction of the exothermic reaction component. The heat generated by the reaction reduced the viscosity of the viscous fluid component to produce a reduced viscosity material, without injecting the viscosity breaker. Using a chandler viscometer, the viscosity of the fracturing fluid, containing the viscous fluid component and the exothermic reaction component, was measured pre-reaction and post-reaction. The viscosity of the fracturing fluid was reduced from 1600 cP to 10 cP, as shown in FIG. 4. The results show that the exothermic reaction component and this type of treatment can clean-up the fractures post a fracturing job.

Example 4

An exothermic reaction component was applied to treat an injection well (also referred to as the wellbore) and the surrounding rock matrix in the Safaniya Oil Field of Saudi Arabia. The injection well was damaged due to the deposition of asphaltenes and corrosion products in the wellbore and in the surrounding rock matrix. A laboratory analysis of the blockage materials showed the main components to be asphaltenes and corrosion products, as shown in Table 1.

TABLE 1 Composition of blockage materials in injection well in Safaniya Oil Field. Component Weight Percent Asphaltene 11.3 Iron Oxides 32.4 Iron Sulfides 21.6 Sodium Chloride 16.5 Calcium Carbonate 9.9 Silica 8.3

FIGS. 5a and 5b are pictorial representations of the blockage materials collected from the injection well. In laboratory testing of the blockage materials, the initial viscosity at 60° F. was about 5,800 cP. The American Petroleum Institute (API) number for the thick oil sludge was 11. The blockage materials were a semi-solid material. The exothermic reaction component reduced the viscosity of the blockage material from its initial viscosity and enabled easy cleanup.

FIG. 6 is a graph showing the effect of the exothermic reaction component on the temperature of blockage materials shown in FIGS. 5a and 5b . As can be seen, once an exothermic reaction component comprising sodium nitrite and ammonium chloride was added to the blockage materials at 127° F. at about the 50 second marker, the temperature increased to over 200° F.

FIG. 7 is a graph showing the effect of an exothermic reaction component on the viscosity of blockage materials in Example 4, shown in FIGS. 5a and 5b . Surprisingly and unexpectedly, the viscosity of the blockage materials (thick oil sludge) was quickly reduced from about 6,000 cP to under 1,000 cP. In the lab-scale experiment, which used 10 grams of oil sludge material, 10 mL of 3M ammonium chloride and 10 ml of 3M sodium nitrite were added to the oil sludge material along with 2 ml of activator (100% acetic acid).

FIG. 8 is a graph showing the effect of an exothermic reaction treatment used to treat the blocked wellbore of Example 4. As shown by the graph, pressure and temperature within the wellbore increase with time. FIG. 9 is a graph showing the effect of an exothermic reaction treatment used to treat the rock matrix surrounding the wellbore of Example 4. As shown by the graph, pressure and temperature within the rock matrix increase with time. Before the treatment of the wellbore and the surrounding rock matrix with an exothermic reaction component, the injectivity of the wellbore was 1,000 barrels of water per day (1.0 MBWD). After the exothermic reaction treatment was used to treat the wellbore and the surrounding rock matrix, the injectivity was improved by about 6 times or about 600% to 5.8 MBPD.

FIG. 10 is a graph showing the pre-exothermic-reaction-component injectivity and post-exothermic-reaction-component injectivity of the injection well of Example 4. As demonstrated in the graph, injectivity was increased greatly over time by the treatment with the exothermic reaction component. WHP is wellhead pressure, and BPM is barrels per minute. Comparing the WHP when pumping 1 BPM before the exothermic reaction component to afterward, injectivity was improved by about 6 times. WHP decreased from about 1,500 psi to about 253 psi. The exothermic reaction component was able to remove asphaltenes, corrosion components, scale, thick oil sludge, and other materials blocking the injection wellbore and surrounding rock matrix.

Injectivity Test #'s 1 and 2 were conducted before the injection of the exothermic reaction component. Injectivity Test #3 was conducted after the addition of 10 bbls of reagent A (sodium nitrite) and 10 bbls of reagent B (ammonium chloride and acetic acid), the reagents A and B prepared as described as follows. Before Test #4, an additional 40 bbls of each reagent A and reagent B were injected into the wellbore, followed by soaking time. Then Test #4 was conducted. Test #'s 5 and 6 were conducted after the addition of another 150 bbls of each reagent A and 150 bbls of reagent B.

To prepare sodium nitrite (reagent A) for the application of the exothermic reaction component to the injection well, first 174 barrels (bbls) of fresh water were placed in a clean tank. Then, 13,165 kilograms (kg) of sodium nitrite, from Bayouni Trading Co. of Saudi Arabia, were added to the fresh water under agitation. Additional fresh water was mixed to balance to a total volume of solution of 200 bbls (however, this step was optional). To prepare ammonium chloride (reagent B) for the application of the exothermic reaction component to the injection well, first 170 bbls of fresh water were placed in a clean tank. Then 10,313 kilograms (kg) of ammonium chloride, from Bayouni Trading Co. of Saudi Arabia, were added to the tank under agitation.

Next, 20 bbls of acetic acid, from Schlumbergeer of Houston, Tex., were added to the tank with the ammonium chloride. In addition, 25 gallons of corrosion inhibitor were added, and fresh water was added to achieve a total volume of solution of 200 bbls (however, this step was optional, and corrosion inhibitor is optional). The components were mixed thoroughly until all components were dissolved.

The concentration of ammonium chloride in solution and sodium nitrite in solution was about 3M. The concentration of acetic acid once mixed with the ammonium chloride solution was about 5% by volume. Using the above method to make sodium nitrite and ammonium chloride solutions, three separate runs were carried out on the well. In a first run, 10 barrels of the sodium nitrite solution and 10 barrels of the ammonium chloride solution were added (before Injectivity Test #3 in FIG. 10). In a second run, 40 barrels of the sodium nitrite solution and 40 barrels of the ammonium chloride solution were added (before Injectivity Test #4 in FIG. 10). In a third run, 150 barrels of the sodium nitrite solution and 150 barrels of the ammonium chloride solution were added (before Injectivity Test #'s 5 and 6 in FIG. 10).

Once the sodium nitrite and ammonium chloride were prepared, the solutions were introduced to the injection well by coiled tubing to produce an exothermic reaction in situ and reduce the viscosity of the blockage materials in the well and surrounding rock matrix. In the present example, HCl was not used as an activator. In some embodiments, HCl can be used as an activator in addition to or alternative to for acid stimulation, however, it was not used during the presently described well treatment.

The exothermic reaction can be activated by acid precursors such as, for example, organic acids like acetic acid and inorganic acids such as hydrochloric acid. Such acid precursors can be encapsulated. Additional precursors can include organic esters.

Additional surprising and unexpected benefits using treatments of the present disclosure are observed. When fracturing low pressure reservoirs, usually fracturing takes a long time to flow back fracturing gel, which may take, for example, about 1-2 weeks. However, when compositions of the present disclosure are used, well head pressure is increased due to generated nitrogen gas in situ. Therefore, this accelerates the flow back time by just bleeding the pressure. As for the above treatment, well head pressure increased, when the compositions of the present disclosure were added from about 2,600 to about 4,400 psi (third run from above, see also FIG. 9). Nitrogen gas generated by the exothermic reaction downhole provides lifting energy to accelerate flow back from the well. Flow back can be reduced from about 1-2 weeks to about 1-2 hours.

Additional and different reactive chemicals that generate additional heat or pressure downhole can be used in addition to or alternative to the exothermic reaction component to enhance the efficiency. Certain example compounds include sodium azide and ammonium nitrite. Such chemicals could be used in addition to or alternative to sodium nitrite, ammonium chloride, and an activator.

Additional reactive chemicals with triggering temperatures at about 300° C. to about 400° C. could be used, where the heat generated by the reaction between ammonium chloride and sodium nitrite would provide activation energy for other reactive chemicals downhole.

Although the present disclosure has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereupon without departing from the principle and scope of the disclosure. Accordingly, the scope of the present disclosure should be determined by the following claims and their appropriate legal equivalents.

The singular forms “a,” “an,” and “the” include plural referents, unless the context clearly dictates otherwise.

Optional or optionally means that the subsequently described event or circumstances can or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed here as from about one particular value, or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value to the other particular value, along with all combinations within said range.

Throughout this application, where patents or publications are referenced, the disclosures of these references in their entireties are intended to be incorporated by reference into this application, in order to more fully describe the state of the art to which the disclosure pertains, except when these references contradict the statements made here.

As used throughout the disclosure and in the appended claims, the words “comprise,” “has,” and “include” and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps.

As used throughout the disclosure, terms such as “first” and “second” are arbitrarily assigned and are merely intended to differentiate between two or more components of an apparatus. It is to be understood that the words “first” and “second” serve no other purpose and are not part of the name or description of the component, nor do they necessarily define a relative location or position of the component. Furthermore, it is to be understood that that the mere use of the term “first” and “second” does not require that there be any “third” component, although that possibility is contemplated under the scope of the present disclosure. 

What is claimed is:
 1. A cleanup fluid for reducing a viscosity of a viscous material in and around a wellbore of a hydrocarbon-bearing formation, the cleanup fluid comprising: an optional acid precursor, the optional acid precursor present at a low concentration not greater than about 5% by volume of the cleanup fluid, if present, to trigger an exothermic reaction component in addition to or alternative to heat in the wellbore, and the acid precursor comprising at least one component selected from the group consisting of: triacetin; methyl acetate; hydrochloric acid; acetic acid; and mixtures thereof; and the exothermic reaction component operable to generate heat, and the exothermic reaction component comprising at least one component selected from the group consisting of: urea; sodium hypochlorite; ammonium chloride; ammonium bromide; ammonium nitrate; ammonium sulfate; ammonium carbonate; ammonium hydroxide; sodium nitrite; potassium nitrite; and mixtures thereof, and the exothermic reaction component comprising at least one ammonium containing compound at about 3 molar (M) concentration and at least one nitrite containing compound at about 3 molar (M) concentration, where the cleanup fluid comprises only liquid components in solution prepared to be fluidly compatible with a viscous fluid component such that the cleanup fluid and viscous fluid component quickly mix and the heat reduces a viscosity of the viscous fluid component to create a reduced viscosity material in less than about 1 minute, and where the cleanup fluid does not include viscosifying components at concentrations that generate foam or introduce foam into the wellbore.
 2. The cleanup fluid of claim 1, where the ammonium containing compound comprises NH₄Cl and the nitrite containing compound comprises NaNO₂.
 3. The cleanup fluid of claim 2, where the concentration of the NH₄Cl and NaNO₂ is about 3 M.
 4. The cleanup fluid of claim 1, where the acid precursor comprises triacetin.
 5. The cleanup fluid of claim 1, where the exothermic reaction component reacts when a temperature within the hydrocarbon-bearing formation reaches about 120° F.
 6. The cleanup fluid of claim 1, further comprising a cross-linked gel.
 7. The cleanup fluid of claim 1, where the viscous fluid component comprises a compound selected from the group consisting of: carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxypropyl cellulose, methyl hydroxyl ethyl cellulose, hydroxypropyl guar, carboxymethyl guar, guar cross-linked boron ions from an aqueous borax/boric acid solution, guar cross-linked with organometallic compounds, aluminum phosphate-ester oil gels, and mixtures thereof.
 8. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 0.1% and about 15% by volume.
 9. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 0.1% and about 10% by volume.
 10. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 1% and about 10% by volume.
 11. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 2% and about 8% by volume.
 12. The cleanup fluid of claim 1, further comprising an aqueous guar solution having a concentration of guar gum between about 4% and about 6% by volume.
 13. The cleanup fluid of claim 1, further comprising an ammonium containing compound selected from the group consisting of: ammonium chloride, ammonium bromide, ammonium nitrate, ammonium sulfate, ammonium carbonate, ammonium hydroxide, and mixtures thereof.
 14. The cleanup fluid of claim 1, further comprising a nitrite containing compound selected from the group consisting of: sodium nitrite, potassium nitrite, and mixtures thereof.
 15. A method for use of a cleanup fluid in a wellbore or reservoir, the method comprising the steps of: optionally preparing an acid precursor, the acid precursor present at a low concentration not greater than about 5% by volume of the cleanup fluid, if present, to trigger an exothermic reaction component, in addition to or alternative to heat in the wellbore or reservoir, and the acid precursor comprising at least one component selected from the group consisting of: triacetin; methyl acetate; hydrochloric acid; acetic acid; and mixtures thereof; preparing the exothermic reaction component operable to generate heat, and the exothermic reaction component comprising at least one component selected from the group consisting of: urea; sodium hypochlorite; ammonium chloride; ammonium bromide; ammonium nitrate; ammonium sulfate; ammonium carbonate; ammonium hydroxide; sodium nitrite; potassium nitrite; and mixtures thereof, and the exothermic reaction component comprising at least one ammonium containing compound at about 3 molar (M) concentration and at least one nitrite containing compound at about 3 molar (M) concentration, where the cleanup fluid comprises only liquid components in solution prepared to be fluidly compatible with a viscous fluid component such that the cleanup fluid and viscous fluid component quickly mix and the heat reduces a viscosity of the viscous fluid component to create a reduced viscosity material in less than about 1 minute, where the cleanup fluid does not include viscosifying components at concentrations that generate foam or introduce foam into the wellbore; injecting the acid precursor, if present, and the exothermic reaction component into the wellbore; allowing the exothermic reaction component to react in situ to produce heat and nitrogen gas, the heat and nitrogen gas operable to increase the temperature and pressure in situ, the viscous material comprising at least one of asphaltenes or corrosion products in the wellbore, to create the reduced viscosity material.
 16. The method of claim 15, where the ammonium containing compound comprises NH₄Cl and the nitrite containing compound comprises NaNO₂.
 17. The method of claim 15, where the exothermic reaction component comprises an ammonium containing compound selected from the group consisting of: ammonium chloride, ammonium bromide, ammonium nitrate, ammonium sulfate, ammonium carbonate, ammonium hydroxide, and mixtures thereof.
 18. The method of claim 15, where the exothermic reaction component comprises a nitrite containing compound selected from the group consisting of: sodium nitrite, potassium nitrite, and mixtures thereof.
 19. The method of claim 15, where the viscous material comprises asphaltenes and corrosion products in the wellbore.
 20. The method of claim 19, where the corrosion products are selected from the group consisting of: iron oxides, iron sulfides, sodium chloride, calcium carbonate, silica, and mixtures thereof.
 21. The method of claim 15, wherein the step of reducing the viscosity of the viscous material to create the reduced viscosity material increases the injectivity of the wellbore.
 22. The method of claim 15, wherein the step of reducing the viscosity of the viscous material to create the reduced viscosity material increases the productivity of the hydrocarbon-bearing reservoir.
 23. The method of claim 15, further comprising the step of increasing nitrogen lift in the wellbore. 